Negative electrode material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery using the same

ABSTRACT

A negative electrode material for a nonaqueous electrolyte secondary battery, where the negative electrode contains a carbon material A and a carbon material B. Carbon material A is a multilayer-structure carbon material containing a graphitic particle having an amorphous carbon surface covering, where the interplaner spacing of 002 planes, by wide-angle XRD, is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm 3  or more, and the Raman R value is from 0.25 to 0.6. Carbon material B is a graphitic particle where the interplanar spacing of 002 planes, by wide-angle XRD, is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm 3  or more, the Raman R value is from 0.2 to 0.5, and the average degree of circularity, determined by a flow-type particle analyzer, is 0.9 or more.

TECHNICAL FIELD

The present invention relates to a carbon material for negative electrode of a nonaqueous electrolyte secondary battery. The present invention also relates to a negative electrode for nonaqueous electrolyte secondary battery using the negative electrode carbon material, and a nonaqueous electrolyte secondary battery having the negative electrode.

BACKGROUND ART

A nonaqueous lithium secondary battery comprising positive and negative electrodes capable of storing/releasing lithium ion and a nonaqueous electrolytic solution having dissolved therein a lithium salt such as LiPF₆ and LiBF₄ has been developed and is used in practice.

Various materials have been proposed as the negative electrode material of this battery, but in view of high capacity and excellent flatness of discharge potential, a graphitic carbon material such as natural graphite, artificial graphite obtained by graphitizing coke or the like, graphitized mesophase pitch, and graphitized carbon fiber is used.

Also, an amorphous carbon material is used because this is relatively stable to some electrolytic solutions. In addition, a carbon material imparted with properties of both graphite and amorphous carbon by coating or attaching amorphous carbon on the surface of a graphitic carbon particle is also used.

In Patent Document 1, a spheroidized graphitic carbon material enhanced in the rapid charge-discharge characteristics by applying a mechanical energy treatment to a graphitic carbon particle that is originally in a flake, scale or plate form, to give a damage to the graphitic particle surface and at the same time, spheroidize the particle is used, and it is further proposed to use a spheroidized carbon material having a multilayer structure, which has the properties of graphite and amorphous carbon by virtue of coating or attaching amorphous carbon on the surface of the spheroidized graphitic carbon particle and simultaneously has rapid charging-discharging property.

However, in recent years, the application of a nonaqueous lithium secondary battery is expanding, and a nonaqueous lithium secondary battery having higher rapid charging-discharging property than ever before and at the same time, having high cycle characteristics is demanded in use for electric power tools, electric cars and the like, in addition to conventional use for notebook-size personal computers, mobile communication equipment, portable cameras, potable game machines and the like.

With respect to improvement of the cycle characteristics, for example, Patent Document 2 has proposed a nonaqueous lithium secondary battery using, as negative electrode materials, a carbonaceous material particle having a multilayer structure, where the R value obtained from the Raman spectrum is 0.2 or more, and an amorphous carbonaceous particle having low crystallinity, where the X-ray interplanar spacing d002 is from 3.36 to 3.60 Å. However, this battery has a problem that the irreversible capacity attributable to the amorphous carbon particle is increased, and more improvements are needed on the cycle characteristics and rapid charge-discharge characteristics required of the recent lithium secondary battery.

Also, Patent Document 3 has proposed a negative electrode material composed of a mixture of a coated graphite particle whose surface is covered with amorphous carbon, and a non-coated graphite particle whose surface is not covered with amorphous carbon.

In the description of Patent Document 3, it is stated that the non-coated graphite particle indicates a graphite particle where the ratio [I1360/I1580] of the peak intensity (I1360) near 1,360 cm⁻¹ to the peak intensity (I1580) near 1,580 cm⁻¹ in argon laser Raman spectrometry at a wavelength of 5,145 Å is 0.10 or less. It is also stated that the coated graphite particle indicates a graphite particle where the ratio [I1360/I1580] of the peak intensity (11360) near 1,360 cm⁻¹ to the peak intensity (I1580) near 1,580 cm⁻¹ in argon laser Raman spectrometry at a wavelength of 5,145 Å is from 0.13 to 0.23.

RELATED ART Patent Document

-   Patent Document 1: Japanese Patent No. 3534391 -   Patent Document 2: Japanese Patent No. 3291756 -   Patent Document 3: JP-A-2005-294011 (the term “JP-A” as used herein     means an “unexamined published Japanese patent application”)

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, the targeted high capacity, rapid charging-discharging property and cycle characteristics sought by the present inventors have not been achieved even by the methods described above.

Accordingly, an object of the present invention is to propose a negative electrode material for nonaqueous electrolyte secondary battery, having high capacity and satisfying both rapid charge-discharge characteristics and high cycle characteristics, which is suited also for the recent application to electric power tools or electric cars.

Means for Solving the Problems

As a result of intensive studies to attain the above-described object, the present inventors have found that when a negative electrode material containing two kinds of carbon materials, that is, carbon material A and carbon material B, differing in the role is used, an electrode for nonaqueous electrolyte secondary battery, satisfying both rapid charge-discharge characteristics and cycle characteristics, can be obtained. The present invention has been accomplished based on this finding.

A material excellent particularly in the rapid charging-discharging property is selected for the carbon material A, and a material excellent particularly in the conductivity is selected for the carbon material B. However, both the carbon material A and the carbon material B must be a carbon material having both high capacity and rapid charge-discharge characteristics.

The carbon material A having both high capacity and rapid charge-discharge characteristics and being excellent particularly in the rapid charge-discharge characteristics is a multilayer-structure carbon material containing a graphitic particle and an amorphous carbon covering the surface thereof, which is a carbon material where the interplanar spacing (d002) of 002 planes of the multilayer-structure carbon material as measured by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, and the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.25 to 0.6.

The carbon material B having both high capacity and rapid discharge characteristics and being excellent particularly in the electron conductivity is a graphitic particle where the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.2 to 0.5, and the average degree of circularity as determined by a flow-type particle analyzer is 0.9 or more.

That is, the present invention is as follows.

1. A negative electrode material for nonaqueous electrolyte secondary battery, comprising the following carbon material A and carbon material B:

(Carbon Material A)

a multilayer-structure carbon material containing a graphitic particle and an amorphous carbon covering the surface of the graphitic particle, which is a carbon material where the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, and the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, is from 0.25 to 0.6,

(Carbon Material B)

a graphitic particle where the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, is from 0.2 to 0.5, and the average degree of circularity as determined by a flow-type particle analyzer is 0.9 or more.

2. The negative electrode material for nonaqueous electrolyte secondary battery as described in 1 above, wherein the ratio in the average particle diameter between the carbon material A and the carbon material B (average particle diameter of carbon material A/average particle diameter of carbon material B) is from 0.7 to 1.3.

3. The negative electrode material for nonaqueous electrolyte secondary battery as described in 1 or 2 above, wherein the ratio of the carbon material Bis from 30 to 70 wt % based on the total amount of the carbon material A and the carbon material B.

4. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 3 above, wherein in the graphitic particle used for the carbon material A, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, and the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.2 to 0.5.

5. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 4 above, wherein the tap density is 0.8 g/cm³ or more.

6. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 5 above, wherein the true density of the carbon material B is 2.21 g/cm³ or more.

7. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 6 above, wherein the specific surface area of the carbon material A is from 0.5 to 8 m²/g.

8. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 7 above, wherein the average degree of circularity of the graphitic particle used for the carbon material A as determined by a flow-type particle analyzer is 0.88 or more.

9. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 8 above, wherein the pore volumes in the range of 10 to 100,000 nm of the carbon material A and the carbon material B as measured by the mercury intrusion method, are 0.4 ml/g or more.

10. The negative electrode material for nonaqueous electrolyte secondary battery as described in any one of 1 to 9 above, wherein the average particle diameter of the carbon material A is from 2 to 30 μM.

11. A negative electrode for nonaqueous electrolyte secondary battery, comprising: a current collector; and a negative electrode layer provided thereon, wherein the negative electrode layer contains the negative electrode material for nonaqueous electrolyte secondary battery described in any one of 1 to 10 and a binder resin.

12. A nonaqueous electrolyte secondary battery comprising the negative electrode described in 11 above, a positive electrode capable of storing/releasing lithium ion, and a nonaqueous electrolytic solution.

Advantage of the Invention

A nonaqueous electrolyte secondary battery using, as an electrode, the negative electrode material for nonaqueous electrolyte secondary battery of the present invention exhibits an excellent property satisfying both rapid charge-discharge characteristics and high cycle characteristics.

MODE FOR CARRYING OUT THE INVENTION

The negative electrode material for nonaqueous electrolyte secondary battery of the present invention (hereinafter, sometimes referred to as the negative electrode material of the present invention) is a negative electrode material for nonaqueous electrolyte secondary battery, containing at least the following carbon material A and carbon material B:

(Carbon Material A)

a multilayer-structure carbon material containing a graphitic particle and an amorphous carbon covering the surface thereof, which is a carbon material where the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, and the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.25 to 0.6,

(Carbon Material B)

a graphitic particle where the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, Lc is 900 Å or more, the tap density is 0.8 g/cm³ or more, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.2 to 0.5, and the average degree of circularity as determined by a flow-type particle analyzer is 0.9 or more.

[Carbon Material A]

(a) Interplanar Spacing (d002) of 002 Planes

In the carbon material A, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, and Lc is 900 Å or more. It is preferred that the interplanar spacing (d002) of 002 planes is 3.36 Å or less and Lc is 950 Å or more. The interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is measured by the method described later in Examples. When the interplanar spacing (d002) of 002 planes is 3.37 Å or less and Lc is 900 Å or more, this indicates that the carbon material has high crystallinity in most portions excluding the surface of a particle thereof and is a carbon material working out to a high-capacity negative electrode material free from capacity reduction due to such a large irreversible capacity as seen in an amorphous carbon material.

(b) Tap Density

The tap density of the carbon material A is 0.8 g/cm³ or more, preferably 0.85 cm³ or more. The tap density is measured by the method described later in Examples. The tap density being 0.8 g/cm³ or more indicates that the carbon material provides a spherical appearance.

If the tap density is less than 0.8 g/cm³, this indicates that the spherical graphitic particle as a raw material of the carbon material A is not formed as an adequately spherical particle, and in this case, a continuous void is not sufficiently ensured in the electrode and the mobility of Li ion in the electrolytic solution held in the void is impaired, giving rise to reduction in the rapid charge-discharge characteristics.

(c) Raman R Value

In the carbon material A, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.25 to 0.6, preferably from 0.25 to 0.5, more preferably from 0.25 to 0.4. The Raman R value is measured by the method described later in Examples.

Studies by the present inventors have revealed that the targeted rapid charge-discharge characteristics are not achieved when using a carbon material in which the Raman R value of the multilayer-structure carbon material for electrode is 0.25 or less.

The Raman R value of the multilayer-structure carbon material is governed by the Raman R value of the graphitic particle and the Raman R value of the covering amorphous carbon coated on the surface of the graphitic carbon. If the Raman R value of the multilayer-structure carbon material is low, this indicates that the Raman R value of the graphitic particle is low and the graphitic particle is not a spheroidized particle having received on the particle surface thereof a sufficient damage by a mechanical energy treatment, and in this case, the amount of sites for receiving or releasing Li ion, such as fine crack, chipping and structural defect on the graphitic particle surface due to damage, is small, giving rise to bad rapid charging-discharging property for Li ion.

Also, if the Raman R value of the carbon material A exceeds 0.6, this indicates that the amount of the amorphous carbon covering the graphitic particle is large, and in this case, the effect on the magnitude of irreversible capacity by the amorphous carbon amount is increased and a small battery capacity results.

When the Raman R is 0.25 or more, this indicates that the carbon material is a multilayer-structure carbon material containing a graphitic particle and an amorphous carbon covering the surface thereof and at the same time, the spherical graphitic particle surface has a fine crack, chipping, a structural defect or the like created due to damage by a mechanical energy treatment.

That is, the carbon material A is a material obtained by a process where a flat graphitic particle is spheroidized while involving folding, winding or chamfering and at the same time, the surface of the graphitic particle having formed on the particle surface thereof a fine crack, chipping, a structural defect or the like is covered with an amorphous carbon.

Also, the carbon material A is a spheroidized particle enjoying high Li ion acceptability of the amorphous carbon and thanks to a fine crack, chipping or a structural defect on the surface of the graphitic particle working as a core, allowing Li ion to easily enter or leave the inside of the graphite crystal, and therefore, this is a carbon material capable of ensuring communicating voids in the electrode and by a synergistic effect with good mobility of Li ion, enhancing the rapid charging-discharging property.

At the same time, the carbon material A is a carbon material having both high capacity by virtue of the particle body being graphitic except for the particle surface, and low irreversible capacity because the amorphous carbon covering the graphitic particle surface produces an effect of suppressing an excessive reaction with an electrolytic solution.

(d) 3R/2H

The wide-angle X-ray diffraction method is used to determine a value indicative of crystallinity of the entire particle, and the argon ion laser Raman spectrum is utilized to determine a value indicative of the surface property of a particle. As for the wide-angle X-ray diffraction method, the measurement is performed by the method described later in Examples. In the carbon material A, the ratio 3R/2H between the intensity 3R(101) of 101 plane based on the rhombohedral crystal structure and the intensity 2H(101) of 101 plane based on the hexagonal crystal structure, as determined by the wide-angle X-ray diffraction method, is preferably 0.1 or more, more preferably 0.2 or more.

The rhombohedral crystal structure is a crystal morphology where a stack of graphite network plane structures is repeated for every three layers. Also, the hexagonal crystal structure is a crystal morphology where a stack of graphite network plane structures is repeated for every two layers. In the case of a graphitic particle showing a crystal morphology where the ratio of the rhombohedral crystal structure 3R is large, Li ion acceptability is high compared with a graphite particle where the ratio of the rhombohedral crystal structure 3R is small.

(e) Specific Surface Area by BET Method

The specific surface area of the carbon material A by the BET method is preferably from 0.5 to 8 m²/g, more preferably from 1 to 6 m²/g, still more preferably from 1.5 to 5 m²/g. The specific surface area by the BET method is measured by the method described later in Examples.

By setting the specific surface area of the carbon material A to 0.5 m²/g or more, the Li ion acceptability is enhanced, and by setting it to 8 m²/g or less, the battery capacity can be prevented from reduction due to increase in the irreversible capacity.

(f) Pore Volume

The pore volume of the carbon material A by the mercury intrusion method in the range of 10 to 100,000 nm is preferably 0.4 ml/g or more, more preferably 0.5 ml/g or more. The pore volume is measured by the method described later in Examples. By setting the pore volume to 0.4 ml/g or more, the area for entering/leaving of Li ion is increased.

(g) Average Particle Diameter

The average particle diameter of the carbon material A is preferably from 2 to 30 μm, more preferably from 4 to 20 μm, still more preferably from 6 to 15 μm. The average particle diameter is measured by the method described later in Examples. By setting the average particle diameter to 2 μm more, the irreversible capacity can be prevented from increasing due to a large specific surface area, and by setting it to 30 μm or less, the rapid charging-discharging property can be prevented from deterioration due to decrease in the contact area of the electrolytic solution with the carbon material A particle.

(h) Interplanar Spacing (d002) of 002 Planes of Graphitic Particle

In the graphitic particle before covering with an amorphous carbon, used in the carbon material A, the interplanar spacing (d002) of 002 planes by the X-ray diffraction method is preferably 3.37 Å or less, and Lc is preferably 900 Å or more. When the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less and Lc is 900 Å or more, this indicates that the carbon material A has high crystallinity in most portions excluding the surface of a particle thereof and is a carbon material working out to a high-capacity electrode free from capacity reduction due to such a large irreversible capacity as seen in an amorphous carbon material.

(i) Tap Density of Graphitic Particle

The tap density of the graphitic particle before covering its surface with an amorphous carbon, used in the carbon material A, is preferably 0.8 g/cm³ or more. When the tap density of the graphitic particle before covering is 0.8 g/cm³ or more, a carbon material satisfying both high capacity and rapid discharge characteristics can be obtained.

(j) Raman K Value of Graphitic Particle

In the graphitic particle before covering with an amorphous carbon, used in the carbon material A, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is preferably from 0.2 to 0.5, more preferably from 0.2 to 0.4.

When the Raman R value is 0.2 or more, this indicates that a fine crack, chipping, a structural defect or the like is created in the graphitic particle surface due to a damage given by applying a mechanical energy treatment in the process of spheroidizing the graphitic particle. Also, when the Raman R value is 0.5 or less, this indicates that the mechanical energy treatment is not so excessive as destroying the crystal structure itself of the graphitic particle and the battery capacity is kept from reduction due to increase in the irreversible capacity caused by destruction of the crystal structure of the graphitic particle.

(k) Average Degree of Circularity of Graphitic Particle

As to the average degree of circularity, in a flow-type particle analyzer capable of individually photographing thousands of particles dispersed in a liquid by using a CCD camera and calculating the average profile parameter, the measurement is performed for particles in the range from 10 to 40 μm by the method described later in Examples. The average degree of circularity is a ratio using the circumferential length of a circle corresponding to the particle area as the numerator and using the circumferential length of a projected particle image photographed as the denominator. As the particle image is closer to a true circle, the value approximates 1, and as the particle image is longer/thinner or more irregular, the value becomes smaller.

The average degree of circularity of the graphitic particle before covering its surface with an amorphous carbon, used in the carbon material A, is preferably 0.88 or more. Also, the graphitic particle is preferably a spheroidized graphitic particle. By setting the average degree of circularity of the graphic particle before covering to 0.88 or more, a carbon material satisfying both high capacity and rapid discharge characteristics can be obtained.

(l) Interplanar Spacing (d002) of 002 Planes of Amorphous Carbon

In the amorphous carbon covering the surface of the graphitic particle, used in the carbon material A, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is preferably 3.40 Å or less, and Lc is preferably 500 Å or more. When the interplanar spacing (d002) of 002 planes is 3.40 Å or less and Lc is 500 Å or more, the Li ion acceptability can be enhanced.

(m) Production Method of Carbon Material A

The carbon material A may be produced without problem by any production method as long as it has the above-described properties, but, for example, a multilayer-structure carbon material for electrode described in Japanese Patent No. 3,534,391 may be used. Specifically, for example, a mechanical energy treatment is applied to a naturally-produced flake-like, scale-like, plate-like or lump-like graphite or an artificial graphite produced by heating petroleum coke, coal pitch coke, coal needle coke, mesophase pitch or the like at 2,500° C. or more, whereby the graphitic particle before covering can be produced.

In the mechanical energy treatment, for example, an apparatus with a rotor having a large number of blades provided inside a casing is used, and the rotor is rotated at a high speed to repeatedly apply a mechanical action such as impact compression, friction and shear force to the natural graphite or artificial graphite introduced into the inside of the rotor, whereby the carbon material can be produced.

The carbon material A can be obtained by mixing the above-described graphitic particle with a petroleum-based or coal-based tar or pitch and a resin such as polyvinyl alcohol, polyacryl nitrile, phenolic resin and cellulose by using, if desired, a solvent or the like, and firing the mixture in a non-oxidizing atmosphere at 500 to 2,500° C., preferably from 700 to 2,000° C., more preferably from 800 to 1,500° C. If desired, pulverization and classification are sometimes performed after the firing.

The coverage that is the amount of the amorphous carbon covering the graphite particle is preferably from 0.1 to 20%, more preferably from 0.2 to 15%, still more preferably from 0.4 to 10%. The coverage can be determined by the method described later in Examples.

By setting the coverage to 0.1% or more, high Li ion acceptability of the amorphous carbon can be fully utilized and good rapid charging property is obtained. Also, by setting the coverage to 20% or less, the capacity reduction due to increase in the effect of the amorphous carbon amount on the magnitude of irreversible capacity can be prevented.

[Carbon Material B]

(a) Interplanar Spacing (d002) of 002 Planes

In the carbon material B, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less, and Lc is 900 Å or more. When the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.37 Å or less and Lc is 900 Å or more, this indicates that the carbon material has high crystallinity in most portions excluding the surface of a particle thereof and is a carbon material working out to a high-capacity electrode free from capacity reduction due to such a large irreversible capacity as seen in an amorphous carbon material,

(b) Tap Density

The tap density of the carbon material B is preferably 0.8 g/cm³ or more. The tap density of the carbon material B being 0.8 g/cm³ or more indicates that the carbon material A provides a spherical appearance.

If the tap density is less than 0.8 g/cm³, this indicates that the spherical graphitic particle is not formed as an adequately spherical particle, and in this case, sufficient communicating voids are not ensured in the electrode and the mobility of Li ion in the electrolytic solution held in the void is impaired, giving rise to reduction in the rapid charge-discharge characteristics.

(c) Raman R Value

In the carbon material B, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is from 0.2 to 0.5, preferably from 0.2 to 0.4.

When the Raman R value is 0.2 or more, this indicates that a fine crack, chipping, a structural defect or the like is created in the graphitic particle surface due to a damage given by applying a mechanical energy treatment in the process of spheroidizing the graphitic carbon particle. Also, when the Raman R value is 0.5 or less, this indicates that the mechanical energy treatment is not so excessive as destroying the crystal structure itself of the graphitic particle and the battery capacity is kept from reduction due to increase in the irreversible capacity caused by destruction of the crystal structure of the graphitic particle.

That is, it is indicated that the carbon material B is a graphitic particle obtained by spheroidizing a flat graphite particle while involving folding, winding or chamfering, and at the same time, creating a fine crack, chipping, a structural defect or the like on the particle surface. Also, it is indicated that the carbon material B is a carbon material enhanced in the rapid charging-discharging property by a synergistic effect between easy entering of Li ion into the graphite bulk crystal thanks to a fine crack, chipping or a structural defect on the particle surface and good Li ion mobility by ensuring voids in the electrode because of a spheroidized particle.

(d) Average Degree of Circularity

The average degree of circularity measured as above of the carbon material B is 0.9 or more. If the average degree of circularity is less than 0.9, the effects of the present invention may not be obtained. The carbon material B is preferably a spheroidized graphitic particle.

(e) 3R/2H

In the carbon material B, the ratio 3R/2H between the intensity 3R(101) of 101 plane based on orientation of a rhombohedral graphite layer and the intensity 2H(101) of 101 plane based on orientation of a hexagonal graphite layer, as determined by the wide-angle X-ray diffraction method, is preferably 0.1 or more, more preferably 0.15 or more. In the case of a graphitic particle showing a crystal morphology where the ratio of the rhombohedral crystal structure 3R is large, Li ion acceptability is high compared with a graphite particle where the ratio of the rhombohedral crystal structure 3R is small.

(f) True Density

The true density of the carbon material B is preferably 2.21 g/cm³ or more, more preferably 2.23 g/cm³ or more, still more preferably 2.25 g/cm³ or more. The true density being 2.21 g/cm³ or more indicates that the main body of the graphite particle has high crystallinity and in turn, the carbon material is a high-capacity carbon material with small irreversible capacity.

(g) Specific Surface Area by BET Method

The specific surface area of the carbon material B by the BET method is preferably from 12 m²/g or less, more preferably 10 m²/g or less, and is preferably 1 m²/g or more, more preferably 1.5 m²/g or more.

By setting the specific surface area of the carbon material B to 12 m²/g or less, the capacity can be prevented from reduction due to increase in the irreversible capacity. Also, by setting the specific surface area to 1 m²/g or more, the area for receiving or releasing lithium is increased and an electrode excellent in rapid charging or rapid discharging property can be obtained.

(h) Pore Volume

The pore volume of the carbon material B by the mercury intrusion method in the range of 10 to 100,000 nm is preferably 0.4 ml/g or more, more preferably 0.5 ml/g or more. By setting the pore volume to 0.4 ml/g or more, the area for entering/leaving of Li ion is increased.

(i) Average Particle Diameter

The average particle diameter of the carbon material B is preferably from 2 to 30 μm, more preferably from 4 to 20 μm, still more preferably from 6 to 15 p.m. By setting the average particle diameter to 2 μm or more, the irreversible capacity can be prevented from increasing due to a large specific surface area, and by setting the average particle diameter to 30 μm or less, the rapid charging-discharging property can be prevented from reduction due to decrease in the contact area of the electrolytic solution with the carbon material B particle.

(j) Production Method of Carbon Material B

The carbon material B may be produced without problem by any production method as long as it has the above-described properties, but, for example, a carbon material for electrode described in Japanese Patent No. 3,534,391 may be used. Specifically, for example, a mechanical energy treatment is applied to a naturally-produced flake-like, scale-like, plate-like or lump-like graphite or an artificial graphite produced by heating petroleum coke, coal pitch coke, coal needle coke, mesophase pitch or the like at 2,500° C. or more, whereby the carbon material can be produced.

In the mechanical energy treatment, for example, an apparatus with a rotor having a large number of blades provided inside a casing is used, and the rotor is rotated at a high speed to apply a mechanical action such as impact compression, friction and shear force to the natural graphite or artificial graphite introduced into the inside of the rotor, whereby the carbon material can be produced.

Unlike the carbon material A, the carbon material B is not a multilayer-structure carbon material. Because, in the carbon material A which is a carbon material enhanced particularly in the rapid charging property by making use of high Li ion acceptability of the amorphous carbon covering the graphitic particle surface, the electron conductivity is disadvantageously reduced due to surface covering with the amorphous carbon, as compared with the graphitic particle before covering.

On the other hand, the carbon material B is a graphitic carbon whose surface is not covered with an amorphous carbon, and therefore, this is a material having higher electron conductivity than the carbon material A, though the rapid charging property is inferior to the carbon material A whose surface is covered with an amorphous carbon. By mixing these carbon material A and carbon material B, an electrode material having both rapid charging-discharging property and high electron conductivity and further showing high cycle characteristics due to a combined effect such as high electrolytic solution retentivity in an interparticle space is obtained.

[Negative Electrode Material for Nonaqueous Electrolyte Secondary Battery]

(a) Mixing Ratio of Carbon Material A and Carbon Material B

The negative electrode material of the present invention is a mixed carbon material containing the above-described carbon material A and carbon material B. In the negative electrode material of the present invention, the ratio of the carbon material B is preferably from 10 to 90 wt %, more preferably from 20 to 80 wt %, still more preferably from 30 to 70 wt %, based on the total amount of the carbon material A and the carbon material B.

By setting the ratio of the carbon material B to 90 wt % or less based on the total amount of the carbon material A and the carbon material B, rapid charging property attributable to the covering amorphous carbon, which is a particularly excellent property of the carbon material A, can be prevented from being impaired and good rapid charge characteristics can be obtained. Also, by setting the ratio of the carbon material B to 10 wt % or more, an electrode fully utilizing the electron conductivity that is a particularly excellent property of the carbon material B can be fabricated and sufficient cycle characteristics are obtained.

(b) Ratio in Average Particle Diameter Between Carbon Material A and Carbon Material B

In the negative electrode material of the present invention, the ratio in the average particle diameter between the carbon material A and the carbon material B (average particle diameter of carbon material A/average particle diameter of carbon material B) is preferably from 0.7 to 1.3, more preferably from 0.8 to 1.2.

If the difference in the average particle diameter between the carbon material A and the carbon material B is large, the particle size distribution of the mixed carbon material becomes wide and when an electrode is formed using the mixed carbon material, the space between particles is filled with particles having a small particle diameter, and the amount of the electrolytic solution held is decreased, making it difficult for Li ion to move.

On the other hand, when the difference in the average particle diameter between the carbon materials mixed is small, that is, when the ratio in the average particle diameter between the carbon material A and the carbon material B is from 0.7 to 1.3, the particle size distribution of the mixed carbon material becomes narrow and the amount of particles as small as filling the space between particles is reduced, so that when an electrode is formed using the carbon material, an adequate interparticle spacing for holding an electrolytic solution can be ensured.

(c) Tap Density

The tap density of the negative electrode material of the present invention is preferably 0.8 g/cm³ or more, more preferably 0.85 g/cm³ or more.

When the tap density of the mixed carbon material B is 0.8 g/cm³ or more, the mixed carbon material can be an adequately spherical particle to assure communicating voids in the electrode and allow for smooth movement of Li ion through the electrolytic solution held in the void and can be a material having excellent rapid charging-discharging property.

[Negative Electrode]

In producing a negative electrode by using the negative electrode material of the present invention, the negative electrode material blended with a binder resin may be made into a slurry with an aqueous or organic medium and after adding a thickening agent, if desired, the slurry may be coated on a current collector and dried.

As the binder resin, a resin that is stable to a nonaqueous electrolytic solution and water-insoluble is preferably used. Examples of the resin which can be used include a rubbery polymer such as styrene, butadiene rubber, isoprene rubber, and ethylene-propylene rubber; a synthetic resin such as polyethylene, polypropylene, polyethylene terephthalate, and aromatic polyamide; a thermoplastic elastomer such as styrene-butadiene-styrene block copolymer or hydrogenated product thereof, styrene-ethylene-butadiene styrene copolymer, and styrene-isoprene styrene block copolymer or hydrogenated product thereof; a soft resinous polymer such as syndiotactic-1,2-polybutadiene, ethylene-vinyl acetate copolymer, and copolymer of ethylene and α-olefin having a carbon number of 3 to 12; and a fluorinated polymer such as polytetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, polypentafluoropropylene, and polyhexafluoropropylene. Examples of the organic medium include N-methylpyrrolidone and dimethylformamide.

The binder resin is preferably used in an amount of usually 0.1 parts by weight or more, preferably 0.2 parts by weight, per 100 parts by weight of the negative electrode material. When the ratio of the binder resin is 0.1 parts by weight or more per 100 parts by weight of the negative electrode material, the binding force between negative electrode materials or between the negative electrode material and the current collector becomes sufficiently high, and decrease in the battery capacity and deterioration of the cycle characteristics can be prevented from occurring due to separation of the negative electrode material from the negative electrode.

Also, the binder resin is preferably used in an amount of usually 10 parts by weight or less, preferably 7 parts by weight or less, per 100 parts by weight of the negative electrode material. When the ratio of the binder resin is 10 parts by weight or less per 100 parts by weight of the negative electrode material, decrease in the capacity of the negative electrode can be prevented and a problem, for example, that lithium ion is blocked from entering/leaving the negative electrode material can be avoided.

As the thickening agent added to the slurry, for example, water-soluble celluloses such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, polyvinyl alcohol and polyethylene glycol may be used. Among these, carboxymethyl cellulose is preferred. The thickening agent is preferably used in an amount of usually from 0.1 to 10 parts by weight, preferably from 0.2 to 7 parts by weight, per 100 parts by weight of the negative electrode material.

As the negative electrode current collector, for example, copper, copper alloy, stainless steel, nickel, titanium and carbon, which are conventionally known to be usable in this application, may be used. The shape of the current collector is usually a sheet shape, and it is also preferred to use a sheet with the surface being made uneven, a net, a punched metal or the like.

After the slurry containing the negative electrode material and a binder resin is coated and dried on a current collector, a pressure is preferably applied to increase the density of the electrode formed on the current collector and make large the battery capacity per unit volume of the negative electrode layer. The density of the electrode is preferably from 1.2 to 1.8 g/cm³, more preferably from 1.3 to 1.6 g/cm³.

By setting the electrode density to 1.2 g/cm³ or more, the capacity of the battery can be prevented from decreasing due to increase in the thickness of the electrode. Also, by setting the electrode density to 1.8 g/cm³ or less, the amount of an electrolytic solution held in a space can be prevented from decreasing due to reduction in the interparticle spacing inside the electrode to impair the mobility of Li ion and deteriorate the rapid charging-discharging property.

[Nonaqueous Electrolyte Secondary Battery]

The nonaqueous electrolyte secondary battery of the present invention can be fabricated according to a conventional method except for using the above-described negative electrode. Examples of the positive electrode material which may be used include a transition metal oxide material such as lithium transition metal composite oxide (for example, a lithium cobalt composite oxide having a basic composition represented by LiCoO₂, a lithium nickel composite oxide represented by LiNiO₂, and a lithium manganese composite oxide represented by LiMnO₂ or LiMn₂O₄) and manganese dioxide, and a composite oxide mixture thereof. Furthermore, TiS₂, FeS₂, Nb₃S₄, Mo₃S₄, CoS₂, V₂O₅, CrO₃, V₃O₃, FeO₂, GeO₂, LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂, and the like may be used.

The positive electrode material blended with a binder resin is made into a slurry with an appropriate solvent, and the slurry is coated and dried on a current collector, whereby the positive electrode can be produced. Incidentally, a conductive material such as acetylene black and Ketjen black is preferably incorporated into the slurry. If desired, a thickening agent may be also incorporated.

As the thickening agent and the binder resin, those well known in this application, for example, those described as examples for use in the production of the negative electrode, may be used. The blending ratio per 100 parts by weight of the negative electrode material is preferably from 0.5 to 20 parts by weight, more preferably from 1 to 15 parts by weight, for the conductive material, and preferably from 0.2 to 10 parts by weight, more preferably from 0.5 to 7 parts by weight, for the thickening agent.

In the case of making a slurry of the binder resin with water, the blending ratio of the binder resin per 100 parts by weight of the positive electrode material is preferably from 0.2 to 10 parts by weight, more preferably from 0.5 to 7 parts by weight, and in the case of making a slurry of the binder resin with an organic solvent capable of dissolving the binder resin, such as N-methylpyrrolidone, the blending ratio is preferably from 0.5 to 20 parts by weight, more preferably from 1 to 15 parts by weight.

Examples of the positive electrode current collector include aluminum, titanium zirconium, hafnium, niobium, tantalum, and an alloy thereof. Among these, aluminum, titanium, tantalum, and an alloy thereof are preferred, and aluminum and an alloy thereof are most preferred.

As the electrolytic solution, a conventionally known electrolytic solution obtained by dissolving various lithium salts in a nonaqueous solvent can be used. Examples of the nonaqueous solvent which may be used include a cyclic carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate, a chain carbonate such as dimethyl carbonate and ethyl methyl carbonate, a cyclic ester such as γ-butyrolactone, a cyclic ether such as crown ether, 2-methyltetrahydrofuran, tetrahydrofuran, 1,2-dimethyltetrahydrofuran and 1,3-dioxolane, and a chain ether such as 1,2-dimethoxyethane, Usually, some of these are used in combination. Above all, a cyclic carbonate and a chain carbonate, or these solvents and further another solvent, are preferably used in combination.

Also, for example, a compound such as vinylene carbonate, vinylethylene carbonate, succinic anhydride, maleic anhydride, propanesultone and diethylsulfone, and a difluorophosphate such as lithium difluorophosphate, may be added. Furthermore, an overcharge inhibitor such as diphenylether and cyclohexylbenzene may be also added.

As the electrolyte that is dissolved in the nonaqueous solvent, for example, LiClO₄, LiPF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(CF₃CF₂SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂) and LiC(CF₃SO₂)₃ may be used. The electrolyte in the electrolytic solution is preferably used at a concentration of usually from 0.5 to 2 mol/liter, preferably from 0.6 to 1.5 mol/liter.

As the separator provided to intervene between the positive electrode and the negative electrode, a porous sheet or nonwoven fabric made of polyolefin such as polyethylene and polypropylene is preferably used.

In the nonaqueous electrolyte secondary battery of the present invention, the volume ratio negative electrode/positive electrode is preferably designed to be from 1.01 to 1.5, more preferably from 1.2 to 1.4.

EXAMPLES

The present invention is described in greater detail below by referring to Examples, but the present invention is not limited to these Examples.

Incidentally, the particle diameter, tap density, specific surface area by BET method, true density, X-ray diffraction, coverage of multilayer-structure carbon powder material, Raman R and the like as referred to in the description of the present invention were measured as follows.

Particle Diameter:

About 20 mg of carbon powder was added to about 1 ml of a 2 (volume) % aqueous solution of polyoxyethylene (20) sorbitan monolaurate, and the mixture was dispersed in about 200 ml of ion exchanged water. The resulting dispersion was measured for the volume-based particle size distribution by using a laser diffraction particle size distribution analyzer (LA-920, manufactured by Horiba Ltd.) to determine the average particle diameter (median diameter), the particle diameter d10 of 10% cumulative part, and the particle diameter d90 of 90% cumulative part. The measurement conditions were ultrasonic dispersion for 1 minute, an ultrasonic intensity of 2, a circulation velocity of 2, and a relative refractive index of 1.50.

Tap Density:

The tap density was measured using a powder density measuring apparatus, Tap Denser KYT-3000 (manufactured by Seishin Enterprise Co., Ltd.). The carbon powder was caused to fall in a 20-cc tap cell through a sieve having a sieve opening of 300 μm to fill up the cell and thereafter, tapping with a stroke length of 10 mm was performed 1,000 times. The density here was taken as the tap density.

Average Degree of Circularity:

The measurement of the particle diameter distribution by the equivalent-circle diameter and the calculation of the degree of circularity were performed using a flow-type particle image analyzer (FPIA-2000, manufactured by To a Medical Electronics Co. Ltd.). Ion exchanged water was used as the dispersion medium, and polyoxyethylene (20) monomethyl laurate was used as the surfactant. The equivalent-circle diameter is a diameter of a circle (equivalent circle) having the same projected area of a particle image photographed, and the degree of circularity is a ratio using the circumferential length of the equivalent circle as the numerator and using the circumferential length of a projected particle image photographed as the denominator. The degrees of circularity of particles in the measured range of 10 to 40 μm were averaged, and the obtained value was taken as the average degree of circularity.

Specific Surface Area by BET Method:

The measurement was performed using AMS-8000 manufactured by Ohkura Riken Co., Ltd. The sample was pre-dried at 250° C. and after further flowing a nitrogen gas for 30 minutes, measured by the BET one-point method based on nitrogen gas adsorption.

Pore Volume (pore volume in the range of 10 to 100,000 nm):

The measurement was performed by a mercury intrusion method using a mercury porosimeter (Autopore 9220, model name, manufactured by Micromeritics Corp.). The sample was sealed in a 5-cc powder cell and pretreated (deaeration) by a mercury porosimeter in vacuum (50.Hg) at room temperature (24° C.) for 10 minutes. Subsequently, the mercury pressure was raised to 40,000 psia from 4.0 psia and then lowered to 15 psia (total number of measurement points: 120). At 120 measurement points, the amount of mercury intruded was measured after an equilibrium time of 5 seconds up to 30 psia and after an equilibrium time of 10 seconds at each subsequent pressure.

The pore distribution was calculated from the thus-obtained mercury intrusion curve by using the Washburn equation (D=−(1/P)4γ cos ψ). Here, D represents the pore diameter, P represents the pressure applied, γ represents the surface tension of mercury (485 dynes/cm was used), and ψ represents the contact angle (140° was used).

True Density:

The true density was measured by using a pyconometer and using, as the medium, a 0.1 wt % aqueous solution of surfactant (polyoxyethylene (20) monolaurate).

X-Ray Diffraction:

A material prepared by adding and mixing X-ray standard high-purity silicon powder in a total amount of about 15 wt % with the carbon powder was measured for a wide-angle X-ray diffraction curve by a reflection diffractometer method using, as the radiation source, CuKα ray monochromatized with a graphite monochrometer. The interplanar spacing (d002) and the crystallite size (Lc) were determined using the Gakushin method, and 3R/2H was determined from the ratio between the intensity 3R(101) of 101 plane based on orientation of the rhombohedral graphite layer and the intensity 2H(101) of 101 plane based on the hexagonal graphite layer.

The interplanar spacing (d002) of 002 planes and Lc by the wide-angle X-ray diffraction method of the amorphous carbon for covering the surface of the spheroidized graphitic particle in the carbon material A were determined on a sample obtained by firing raw material tar pitch alone (not mixed with graphite particle) at a predetermined temperature and pulverizing the fired material.

Coverage of Complex-Structure Carbon Material:

The coverage was determined according to the following formula. Coverage (wt %)=100−(K×D)/((K+T)×N)×100 wherein K represents the weight (Kg) of spherical graphitic carbon used for mixing with tar pitch, T represents the weight (kg) of tar pitch as a covering raw material used for mixing with spherical graphitic carbon, D represents the amount of a mixture actually used for firing out of the mixture of K and T, and N represents the weight of the coated spherical graphitic carbon material after firing. Raman Measurement:

Using NR-1800 manufactured by JASCO Corp., the intensity IA of the peak PA near 1,580 cm⁻¹ and the intensity IB of the peak PB in the range of 1,360 cm⁻¹ were measured in a Raman spectrum analysis where argon ion laser light at a wavelength of 514.5 nm was used, and the intensity ratio R=IB/IA was determined. The sample was prepared by causing a material in a powder state to freely fall in and fill the cell, and the measurement was performed by irradiating the sample surface in the cell with laser light while rotating the cell within a plane perpendicular to the laser light,

Press Load:

An electrode coated in a width of 5 cm was adjusted to the objective density by pressing with a load cell-attached roll of 250 m in diameter. The load required here was measured by the load cell, and the value obtained was taken as the press load.

Example 1 Production of Carbon Material A

A flake-like graphite particle which is naturally-produced graphite and in which the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.36 Å, Lc is 1,000 Å or more, the tap density is 0.46 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is 0.13, the average particle diameter is 28.7 μm, and the true density is 2.27 g/cm³, was used as the raw material graphite.

The flake-like graphite particle was continuously treated using a hybridization system manufactured by Nara Machinery Co., Ltd. under the conditions of a rotor peripheral velocity of 60 in/sec, 10 minutes and a treatment rate of 20 kg/hour, whereby a spheroidization treatment was performed while giving a damage to the graphite particle surface. Thereafter, fine powder particles were removed by a classification treatment.

In this spheroidized graphitic carbon, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.83 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.24, the average particle diameter was 11.6 μm, the specific surface ratio by BET method was 7.7 m²/g, the true density was 2.27 g/cm³, and the average degree of circularity was 0.909.

Subsequently, 100 parts by weight of the spheroidized graphitic carbon obtained above and 9.4 parts by weight of coal tar pitch were mixed under heating at 160° C. in a mixing machine, and the mixture was fired up to 1,000° C. over 2 weeks in a non-oxidizing atmosphere, then cooled to room temperature, and subjected to pulverization and classification, whereby a multilayer-structure spheroidized carbon material was obtained.

In this multilayer-structure spheroidized carbon material, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.98 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.31, the average particle diameter was 11.6 μm, the particle diameter d10 was 7.6 μm, the particle diameter d90 was 17.5 μm, the specific surface ratio by BET method was 3.5 m²/g, the coverage was 5.0%, the ratio 3R/2H between the rhombohedral 3R and the hexagonal 2H by the wide-angle X-ray diffraction method was 0.26, and the pore volume in the range of 10 to 100,000 nm was 0.74 ml/g.

Separately, only the coal tar pitch used above was fired up to 1,300° C. in a nitrogen atmosphere, then cooled to room temperature and pulverized. In the obtained amorphous carbon alone, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.45 Å, and Lc was 24 Å,

Production of Carbon Material B

A flake-like graphite particle which is naturally-produced graphite and in which the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.36 Å, Lc is 1,000 Å or more, the tap density is 0.46 g/cc, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is 0.13, the average particle diameter is 28.7 μm, and the true density is 2.27 g/cm³, was used as the raw material graphite.

The flake-like graphite particle was continuously treated using a hybridization system manufactured by Nara Machinery Co., Ltd. under the conditions of a rotor peripheral velocity of 60 m/sec, 8 minutes and a treatment rate of 20 kg/hour, whereby a spheroidization treatment was performed while giving a damage to the graphite particle surface.

In this spheroidized graphitic carbon, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.96 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.22, the average particle diameter was 13.4 μm, the particle diameter d10 was 9.3 μm, the particle diameter d90 was 19.7 μm, the specific surface ratio by BET method was 7.8 m²/g, the true density was 2.27 g/cm³, the average degree of circularity was 0.908, the ratio 3R/2H between the rhombohedral 3R and the hexagonal 2H by the wide-angle X-ray diffraction method was 0.20, and the pore volume in the range of 10 to 100,000 nm was 0.68 ml/g.

(Production of Mixed Carbon Material)

The carbon material A and the carbon material B obtained above were mixed such that the ratio of the carbon material B became 30 wt % based on the total amount of the carbon material A and the carbon material B, whereby a mixed carbon material was obtained. The tap density of the mixed carbon material was 0.98 g/cm². The ratio of the average particle diameter of the carbon material A to the average particle diameter of the carbon material B was 11.6 m/13.2 μm=0.88.

(Production of Battery for Performance Evaluation)

100 Parts by weight of the mixed carbon material above was added with 100 parts by weight of a 1 wt % aqueous solution of carboxymethyl cellulose and 2 parts by weight of a 50 wt % water dispersion of styrene butadiene rubber and kneaded to make a slurry. This slurry was coated at a basis weight of 4.4 mg/cm² on a copper foil by a doctor blade method, then dried at 110° C. and consolidated by a roll press to give a density of 1.40 g/cc. The press load required for this consolidation was 270 kg. The resulting electrode was cut out to a 32 mm×42 mm rectangle and dried under reduced pressure at 190° C. to obtain a negative electrode.

85 Parts by weight of lithium-nickel-manganese-cobalt-based composite oxide powder was added with 10 parts by weight of carbon black, 41.7 parts by weight of a 12 wt % N-methylpyrrolidone solution of polyvinylidene fluoride, and an appropriate amount of N-methylpyrrolidone and kneaded to make a slurry, and this slurry was coated at a basis weight of 8.8 mg/cm² on an aluminum foil by a doctor blade method, dried at 110° C., consolidated by a roll press to give a positive electrode layer density of 2.45 g/cm³, then cut out to a 30 mm×40 mm rectangle, and dried at 140° C. to obtain a positive electrode.

The negative electrode and positive electrode obtained above were superposed through a separator impregnated with an electrolytic solution to produce a battery for a charge-discharge test. As the electrolytic solution, a solution prepared by dissolving LiPF₆ in a mixed solution of ethylene carbonate:dimethyl carbonate:ethyl methyl carbonate=3:3:4 (by weight) to have a concentration of 1 mol/liter was used.

This battery was subjected to initial adjustment by repeating twice an operation of charging to 4.1 V at 0.2 C, further charging until 0.1 mA with 4.1 V, then discharging to 3.0 V at 0.2 C, subsequently charging to 4.2 V at 0.2 C, further charging until 0.1 mA with 4.2 V, and thereafter discharging to 3.0 V at 0.2 C.

(Evaluation of Rapid Discharging Property)

After charging to 4.2 V at 0.2 C (charging in 5 hours) and further charging for 2 hours with 4.2 V (0.2 C-CCCV), a discharge test to 3.0 V at 0.2 C (discharging in 5 hours), 1 C (discharging in 1 hour), 2 C (discharging in 0.5 hours), 5 C (discharging in 0.2 hours), 10 C (discharging in 0.1 hours) or 15 C (discharging in 0.07 hours) was performed, and the discharge capacity at each rate based on the discharge capacity at 0.2 C (discharging in 5 hours) was expressed in %. The results are shown in Table 1.

(Evaluation of Rapid Charging Property)

A charge test of charging to 4.2 V at 0.2 C (charging in 5 hours), further charging for 2 hours with 4.2 V (0.2 C-CCCV), and charging to 4.2 V at 0.2 C (charging in 5 hours), 1 C (charging in 1 hour), 2 C (charging in 0.5 hours), 5 C (charging in 0.2 hours), 8 C (charging in 0.13 hours) or 10 C (charging in 0.1 hours) was performed, and the charge capacity in each charge test based on the charge capacity when charged to 4.2 V at 0.2 C (charging in 5 hours) and further charged for 2 hours with 4.2 V (0.2 C-CCCV) was expressed in %. The results are shown in Table 1. Incidentally, discharging to 3.0 V at 0.2 C was performed after each charging.

(Evaluation of Cycle Characteristics)

Charging to 4.2 V at 2 C and discharging to 3.0 V at 3 C of the battery above were repeated, and the discharge capacities at 300th cycle and 500th cycle based on the discharge capacity at 1st cycle were expressed in % as a 300-cycles sustaining ratio and a 500-cycles sustaining ratio, respectively. The results are shown in Table 1.

Example 2

This was performed in the same manner as in Example 1 except for changing the ratio of the carbon material B to 50 wt % based on the total amount of the carbon material A and the carbon material B. The results are shown in Table 1.

Comparative Example 1

This was performed in the same manner as in Example 1 except for not mixing the carbon material B with the carbon material A. The results are shown in Table 1.

Comparative Example 2 Production of Carbon Material A

A flake-like graphite particle which is naturally-produced graphite and in which the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method is 3.36 Å, Lc is 1,000 Å or more, the tap density is 0.46 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum is 0.13, the average particle diameter is 28.7 μm, and the true density is 2.27 g/cm³, was used as the raw material graphite.

The flake-like graphite particle was continuously treated using a hybridization system manufactured by Nara Machinery Co., Ltd. under the conditions of a rotor peripheral velocity of 60 m/sec, 10 minutes and a treatment rate of 20 kg/hour, whereby a spheroidization treatment was performed while giving a damage to the graphite particle surface. Thereafter, fine powder particles were removed by a classification treatment.

In this spheroidized graphitic carbon, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.83 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.24, the average particle diameter was 11.6 μm, the specific surface ratio by BET method was 7.7 m²/g, the true density was 2.27 g/cm³, and the average degree of circularity was 0.909.

Subsequently, 100 parts by weight of the spheroidized graphitic carbon obtained above and 10 parts by weight of petroleum-derived pitch were charged into Loedige mixer manufactured by MATSUBO Corporation, kneaded at 90° C., thereafter fired up to 1,300° C. over 2 hours in a nitrogen atmosphere, held for 2 hours, then cooled to room temperature, and further subjected to pulverization and classification, whereby a multilayer-structure spheroidized carbon material was obtained.

In this multilayer-structure spheroidized carbon material, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.99 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.22, the average particle diameter was 11.6 μm, the particle diameter d10 was 7.7 the particle diameter d90 was 17.5 μm, the specific surface ratio by BET method was 3.8 m²/g, the true density was 2.27 g/cm³, the coverage was 3.0%, the ratio 3R/2H between the rhombohedral 3R and the hexagonal 2H by the wide-angle X-ray diffraction method was 0.24, and the pore volume in the range of 10 to 100,000 nm was 0.62 ml/g.

Separately, only the coal-derived pitch used above was fired up to 1,300° C. in a nitrogen atmosphere, then cooled to room temperature and pulverized. In the obtained amorphous carbon alone, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.45 Å, and Lc was 22 Å.

Others were performed in the same manner as in Comparative Example 1. The results are shown in Table 1.

Comparative Example 3 Carbon Material A

The same carbon material as the carbon material A in Comparative Example 2 was used.

Carbon Material B

A commercially available graphitic particle, SFG44, produced by TIMCAL was used. In this SFG44, the interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method was 3.36 Å, Lc was 1,000 Å or more, the tap density was 0.44 g/cm³, the Raman R value that is a ratio of the peak intensity near 1,360 cm⁻¹ to the peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum was 0.10, the average particle diameter was 25.4 μn, the particle diameter d10 was 7.5 μm, the particle diameter d90 was 46.5 μm, the specific surface ratio by BET method was 4.4 m²/g, the true density was 2.27 g/cm³, the average degree of circularity was 0.84, the ratio 3R/2H between the rhombohedral 3R and the hexagonal 2H by the wide-angle X-ray diffraction method was 0.28, and the pore volume in the range of 10 to 100,000 nm was 1.30 ml/g.

(Production of Mixed Carbon Material)

The carbon material A and the carbon material B obtained above were mixed such that the ratio of the carbon material B became 30 wt % based on the total amount of these carbon materials, whereby a mixed carbon material was obtained. The tap density of the mixed carbon material was 0.82 g/cm³. The ratio of the average particle diameter of the carbon material A to the average particle diameter of the carbon material B was 11.6 m/25.4 μm=0.46. Others were performed in the same manner as in Example 1. The results are shown in Table 1.

TABLE 1 Comparative Comparative Comparative Items Unit Example 1 Example 2 Example 1 Example 2 Example 3 Properties of Carbon Material A d002 Å 3.36 3.36 3.36 3.36 3.36 Lc Å >1000 >1000 >1000 >1000 >1000 Raman R 0.31 0.31 0.31 0.22 0.22 Tap density g/cm³ 0.98 0.98 0.98 0.99 0.99 3R/2H 0.26 0.26 0.26 0.24 0.24 Average particle diameter μm 11.6 11.6 11.6 11.6 11.6 Particle diameter d10 μm 7.6 7.6 7.6 7.7 7.7 Particle diameter d90 μm 17.5 17.5 17.5 17.5 17.5 BET Specific surface area m²/g 3.5 3.5 3.5 3.8 3.8 Pore volume ml/g 0.74 0.74 0.74 0.62 0.62 Coverage % 5.0 5.0 5.0 3.0 3.0 Average degree of circularity of 0.909 0.909 0.909 0.909 0.909 0.909 spheroidized graphitic particle before covering d002 of covering amorphous carbon alone Å 3.45 3.45 3.45 3.45 3.45 Lc of covering amorphous carbon alone Å 24 24 24 22 22 Properties of Carbon Material B d002 Å 3.36 3.36 — — 3.36 Lc Å >1000 >1000 — — >1000 Raman R 0.22 0.22 — — 0.10 Tap density g/cm³ 0.96 0.96 — — 0.44 3R/2H 0.20 0.20 — — 0.28 Average particle diameter μm 13.4 13.4 — — 25.4 Particle diameter d10 μm 9.3 9.3 — — 7.5 Particle diameter d90 μm 19.7 19.7 — — 19.7 BET Specific surface area m²/g 7.8 7.8 — — 4.4 Pore volume ml/g 0.68 0.68 — — 1.30 True density g/cm³ 2.27 2.27 — — 2.27 Average degree of circularity 0.908 0.908 — — 0.840 Mixed Carbon Material Ratio of carbon material B based on total wt % 30 50 0 0 30 amount of carbon material A and carbon material B Average particle diameter ratio between 0.88 0.88 — — 0.46 carbon material A and carbon material B Tap density g/cm³ 0.98 0.99 — — 0.82 Press load kg 270 210 320 400 26 Rapid Discharge Characteristics 0.2 C Discharge capacity mAh 13.6 13.1 13.4 13.5 13.4 1 C Discharge/0.2C discharge % 95 94 96 95 94 2 C Discharge/0.2C discharge % 92 90 92 92 90 5 C Discharge/0.2C discharge % 86 84 86 86 80 10 C Discharge/0.2C discharge % 70 74 71 68 65 15 C Discharge/0.2C discharge % 32 41 35 22 20 Rapid Charge Characteristics 0.2 C-CCCV Charge capacity mAh 13.3 13.7 13.6 13.3 13.4 0.2 C Charge/0.2C-CCCV Charge % 98 98 98 97 97 1 C Charge/0.2C-CCCV Charge % 96 96 96 93 93 2 C Charge/0.2C-CCCV Charge % 94 94 94 90 88 5 C Charge/0.2C-CCCV Charge % 87 86 86 81 78 8 C Charge/0.2C-CCCV Charge % 73 70 74 64 60 10 C Charge/0.2C-CCCV Charge % 52 57 55 40 35 Cycle characteristics 300-Cycles sustaining ratio % 59 90 36 20 25 500-Cycles sustaining ratio % 42 88 30 15 20

As seen in Table 1, in Examples 1 and 2 using the negative electrode material according to the present invention, excellent rapid charging-discharging properties and cycle characteristics were exhibited. On the other hand, in Comparative Example using a negative electrode material out of the scope of the present invention, the cycle characteristics were poor, and in Comparative Examples 2 and 3, the rapid charging-discharging properties and the cycle characteristics were low.

INDUSTRIAL APPLICABILITY

The nonaqueous electrolyte secondary battery comprising an electrode using the negative electrode material of the present invention exhibits excellent properties satisfying both rapid charge-discharge characteristics and high cycle characteristics.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

This application is based on Japanese Patent Application (Patent Application No. 2009-079945) filed on Mar. 27, 2009, the entirety of which is incorporated herein by way of reference. 

The invention claimed is:
 1. A negative electrode material for nonaqueous electrolyte secondary battery, comprising a carbon material (A) and a carbon material (B), wherein the carbon material (A) is a multilayer-structure carbon material comprising a graphitic particle and an amorphous carbon covering the surface of the graphitic particle, and wherein the carbon material (A) has: an interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method of 3.37 Å or less; a crystallite size (Lc) of 900 Å or more; a tap density of 0.8 g/cm³ or more; and a Raman R value, as a ratio of a peak intensity near 1,360 cm⁻¹ to a peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, from 0.25 to 0.6, and wherein the carbon material (B), which is not a multilayer-structure carbon material, comprises a graphitic particle, and wherein the carbon material (B) has: an interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method of 3.37 Å or less; a crystallite size (Lc) of 900 Å or more; a tap density of 0.8 g/cm³ or more; a Raman R value, as a ratio of a peak intensity near 1,360 cm⁻¹ to a peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, from 0.2 to 0.5; and an average degree of circularity, as determined by a flow-type particle analyzer, of 0.9 or more.
 2. The negative electrode material of claim 1, wherein a ratio in the average particle diameter between the carbon material (A) and the carbon material (B) (average particle diameter of carbon material (A)/average particle diameter of carbon material (B)) is from 0.7 to 1.3.
 3. The negative electrode material of claim 1, wherein a ratio of the carbon material (B) is from 30 to 70 wt %, based on a total amount of the carbon material (A) and the carbon material (B).
 4. The negative electrode material of claim 1, wherein in the graphitic particle employed for the carbon material (A) has: an interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method of 3.37 Å or less; a crystallite size (Lc) of 900 Å or more; a tap density of 0.8 g/cm³ or more; and a Raman R value, as a ratio of a peak intensity near 1,360 cm⁻¹ to a peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, from 0.2 to 0.5.
 5. The negative electrode material of claim 1, having a tap density of 0.8 g/cm³ or more.
 6. The negative electrode material of claim 1, wherein the carbon material (B) has a true density of 2.21 g/cm³ or more.
 7. The negative electrode material of claim 1, wherein a specific surface area of the carbon material (A) is from 0.5 to 8 m²/g.
 8. The negative electrode material of claim 1, wherein the average degree of circularity of the graphitic particle employed for the carbon material (A), as determined by a flow-type particle analyzer, is 0.88 or more.
 9. The negative electrode material of claim 1, wherein the pore volumes in the range of 10 to 100,000 nm of the carbon material (A) and the carbon material (B), as measured by the mercury intrusion method, are 0.4 ml/g or more.
 10. The negative electrode material of claim 1, wherein an average particle diameter of the carbon material (A) is from 2 to 30 μm.
 11. A negative electrode for nonaqueous electrolyte secondary battery, comprising: a current collector; and a negative electrode layer disposed on the current collector, wherein the negative electrode layer comprises the negative electrode material of claim 1 and a binder resin.
 12. A nonaqueous electrolyte secondary battery, comprising: the negative electrode of claim 11; a positive electrode, which stores and releases lithium ion; and a nonaqueous electrolytic solution.
 13. The negative electrode material of claim 1, wherein a ratio of the carbon material (B) is from 10 to 90 wt %, based on a total amount of the carbon material (A) and the carbon material (B).
 14. The negative electrode material of claim 1, wherein the ratio in the average particle diameter between the carbon material (A) and the carbon material (B) is from 0.8 to 1.2.
 15. The negative electrode material of claim 1, wherein a ratio of the carbon material (B) is from 30 to 50 wt %, based on a total amount of the carbon material (A) and the carbon material (B).
 16. The negative electrode material of claim 1, wherein in the graphitic particle employed for the carbon material (A) has: an interplanar spacing (d002) of 002 planes by the wide-angle X-ray diffraction method of 3.36 Å or less; a crystallite size (Lc) of 950 Å or more; a tap density of 0.85 g/cm³ or more; and a Raman R value, as a ratio of a peak intensity near 1,360 cm⁻¹ to a peak intensity near 1,580 cm⁻¹ in the argon ion laser Raman spectrum, from 0.25 to 0.5.
 17. The negative electrode material of claim 5, having a tap density of 0.85 g/cm³ or more.
 18. The negative electrode material of claim 6, wherein the true density of the carbon material (B) is 2.23 g/cm³ or more.
 19. The negative electrode material of claim 7, wherein the specific surface area of the carbon material (A) is from 1 to 6 m²/g.
 20. The negative electrode material of claim 9, wherein the pore volumes in the range of 10 to 100,000 nm of the carbon material (A) and the carbon material (B), as measured by the mercury intrusion method, are 0.5 ml/g or more. 